Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1a, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bond connections 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 containing a wavelength conversion material such as a phosphor so that light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces).
A conventional LED package 20 illustrated in FIG. 1b may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wire bond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
Current LED packages (e.g. XLamp™ LEDs provided by Cree, Inc.) incorporate one LED chip and higher light output is achieved at the assembly level by mounting several of these LED packages onto a single circuit board. FIG. 2 shows a sectional view of one such distributed integrated LED array 30 comprising a plurality of LED packages 32 mounted to a substrate or submount 34 to achieve higher luminous flux. Typical arrays include many LED packages, with FIG. 2 only showing two for ease of understanding. Alternatively, higher flux components have been provided by utilizing arrays of cavities, with a single LED chip mounted in each of the cavities. (e.g. TitanTurbo™. LED Light Engines provided by Lamina, Inc.)
These LED array solutions may be less compact than desired, as they include extended non-light emitting “dead space” between adjacent LED packages. This dead space can result in larger devices, and can provide for non-light emitting structures that can absorb light and reduce the total luminous flux of the LED package. The above solutions present challenges in providing a compact LED lamp structure incorporating an LED component that delivers light flux levels in the 1000 Lumen and higher range from a small optical source. Moreover, to achieve desired beam shapes, individual optical lenses are typically mounted with each LED component, or very large reflectors (larger than the form of existing conventional sources) have to be employed. These secondary optical elements (lenses or reflectors) are large and costly, and any light being reflected from the sidewalls in the packages and cavities can also result in additional optical losses, making these overall LED components less efficient. As a result, the luminance of a LED package is significantly affected by its package structure.
It is also generally observed that LED's perform best when operating temperatures are minimized. Thus, it is generally desirable to remove heat from the LED, typically by heat transfer via the substrate or submount. One of the best ceramic substrates for heat transfer is aluminum nitride (AlN). However, at least one problem with AlN as a heat transfer material in a LED package is that it is dark brown in color upon deposition, which absorbs visible light and reduces the total luminous flux of the package. Conventional technology is to cover as much of the heat transfer material and/or dead space areas with reflective metal, or with white soldermask to maximize reflectivity while at the same time providing heat transfer. Unfortunately, metal cannot be applied everywhere in high density LED packages due to its electrical conductive properties. Typically, a 75-150 micron gap between areas of different potential in such packages is provided, which results in significant total dead space area having, for example, dark brown AlN in proximity to the light emitting elements. Soldermask is widely used because it is photo-imageable, or screen printable, but the material properties and application methods preclude its use in all conditions. White soldermask also discolors after solder reflow or with time and with photon exposure adding to the other existing problems of lumen loss and color shift. There is also a significant amount of area (e.g., known as “canyon walls”) between light emitting elements that also absorb or poorly reflect the luminous light. These conventional solutions are, for the most part, inadequate for maximizing the total luminous flux of a solid state lighting package.